In order to impart properties such as decorative properties, anti-weathering properties, surface conductivity for antistatic purposes and the like, electromagnetic shielding properties, antibiotic functions, and shock resistance, to articles, metallic films have been formed on the surface of the articles heretofore. Metallic films can be formed by various methods; among them, methods for forming electroplating films by means of electroplating processes are widely employed in practice because they are also suitable for mass production.
However, in order to form electroplating films on the surface of articles, it is required that the surface of the articles possesses electric conductivity. Hence, electroplating films cannot be directly formed on the surface of an article made of a non-conductive material such as plastics, wood, papers, glass, ceramics, rubbers, and concrete. Furthermore, there are cases in which metallic films are required to be formed on the surface of an article made of a metallic material such as magnesium, aluminum, and titanium, (e.g., housings of cellular phones, laptop personal computers, etc.), however, for example, magnesium is one of the most base metals. Thus, in case an attempt is made to form an electroplating film on the surface of such an article, an abrupt substitution plating reaction occurs at the instant of immersing the article in the plating bath, and this makes the formation of high quality electroplating films unfeasible. Aluminum and titanium are metals that are easily oxidized, and in general, the surface of such metals is covered with extremely dense metal oxide films. Accordingly, although these metals are lower in ionization tendency, the surface potential is elevated to make an electroplating treatment difficult. While it is possible to form electroplating films by removing the metal oxide films from the surface, a special etching technology is needed, and there still remain practical problems due to time constraints, because an electroplating process should be carried out before the metal oxide film is formed again after removing the metal oxide films. Furthermore, there may be employed a method of performing an electroplating process comprising, carrying out the so-called zincate treatment for forming a zinc substituted plating film, while simultaneously applying etching, under a strong alkaline environment by immersing the article in a solution containing sodium hydroxide and zinc hydroxide, and then carrying out the step of forming an electroless plating film, and then carrying out the electroplating process. However, this makes the entire process complicated.
Furthermore, in case of forming a uniform electroplating film on the surface of an article having pores, fine grooves, or irregularities on the surface thereof, such as wooden bats, bricks, die-castings, and the like, there remain problems to be solved; considerations should be made on not only how to impart electric conductivity to the surface of the article, but also how to ensure surface smoothness of the article.
Moreover, a corrosion of an article may occur on carrying out an electroplating process in case of an article made of a highly corrosive material such as metallic magnesium; hence, difficulties are found on forming electroplating films on such articles.
In case of solving the above problems by means of known technologies, there may be employed a method as disclosed in Japanese Patent Laid-Open No. 210183/1986, comprising forming, on the surface of the article, a resin coating made of a resin containing dispersed therein a metallic powder, and then forming an electroless plating film on the surface of the resin coating; an electroplating film can be formed on the thus formed surface of the electroless plating film. However, since an electroless plating film is formed by reacting reducing agents to metallic ions in the plating solution and obtaining metal precipitates as a consequence on the surface of plated articles, not only the adhesiveness to the plated object, but also the film deposition efficiency are poor. Although the film deposition efficiency can be increased by methods using palladium catalysts or platinum catalysts, these methods inevitably increase costs. Furthermore, there is no denying that impurities contained in an electroless plating film and originated from the reducing agents provide negative influences on the formation of electroplating films on the surface of the electroless plating film.
Accordingly, an object of the invention is to provide a method for forming a uniform and dense electroplating film with high adhesion strength on the surface of an article, yet irrespective of the surface material and the surface properties of the article.